There are four main objectives of this proposal: 1) An electrophysiological investigation of the convergence of vestibular, optokinetic and neck proprioceptive information at the caudal inferior olive and at the flocculus and nodulus of the rabbit cerebellum. Single unit recordings will be obtained from olivary neurons and Purkinje cells during visual, vestibular, and neck-proprioceptive stimulation testing the idea that the low discharge rate of cerebellar climbing fibers encodes sensory stimulation of low temporal frequency, and that this information is used to modulate eye movements of low velocity. These experiments will also evaluate interactions between climbing fiber responses (CFRs) and simple spikes (SSs) at the level of the cerebellar cortex; testing the idea that Golgi cell inhibition accounts, in part, for the antiphasic behavior of CFRs and SSs. 2) Behavioral description of the development of negative optokinetic afternystagmus in the rabbit. Negative OKAN (slow phase opposite to the inducing optokinetic stimulus) is evoked following prolonged (12-144 h) optokinetic stimulation. The development and persistence of negative OKAN will be studied parametrically. The influence of electrolytic lesions of the dorsal cap on the induction of negative OKAN will be measured. The spontaneous activity of vestibular primary afferent fibers in rabbits which evince negative OKAN will be measured. This will test the idea that negative OKAN reflects a vestibular efferent induced imbalance in primary afferent activity. 3) Histochemical description of the inferior olive, vestibular and vestibulo-cerebellum. The distribution of GAD, a GABA synthetic enzyme, in the inferior olive will be examined. The source of GABAergic pathways to the inferior olive will be studied using lesion, retrograde and orthograde tracer techniques. 4) Modification of vestibular primary afferent activity as a consequence of plugging operations of the semicircular canals. Microelectrode recordings will be made from single vestibular primary afferent fibers before and after insertion and removal of horizontal or anterior semicircular canal plugs. These recordings will test the reversibility of the semicircular canal plugging procedure.